66 Prof. Sedgwick on the general Structure 



tain limestone are perfectly crystalline ; — ^and the large globular structure of 

 magnesian limestone has occasionally been superinduced on masses originally 

 deposited in thin laminje; as is demonstrated by the fact^ that the lines of the 

 original laminae may still, in some instances, be traced through a congeries of 

 balls mutually compressing each other. These peculiarities of structure are 

 found in places where there is not the least indication of any igneous action. 



3. The highest greywacke group of the Cumbrian mountains contains beds 

 with numerous organic remains. The next inferior group (composed of green 

 slate and porphyry), and the third group in the descending order (composed 

 of fine, black clay slate and greywacke slate, &c.), contain no organic remains. 

 How can we account for the absence of organic remains in this third group, 

 except on the supposition that they did not exist in the seas where it was de- 

 posited ? It is developed through large mountain tracts, and is, with very few 

 exceptions, composed of rocks unaltered by igneous action. Had there been 

 any organic beings where it was deposited, w^e should, I think, have found 

 their remains imbedded in it. We arrive at precisely the same conclusions in 

 traversing the whole slate system of Devonshire and Cornwall. The upper 

 part contains organic remains ; the lower part does not. Yet in the lower 

 part there are large tracts where the rocks are unaltered, and contain beds 

 of limestone and calcareous slate, not more crystalline than ordinary beds 

 of mountain limestone. The same positive and negative conclusions are 

 strengthened by the phenomena of North and South Wales. I may state 

 generally, as the result of my own observations on the older strata of this 

 Island, that there is a line in the descending series where organic remains 

 seem entirely to disappear; and that this line is by no means coordinate with 

 mineral changes superinduced by igneous action. It is true that igneous 

 action has in many cases produced a crystalline structure in stratified masses, 

 and obliterated their organic remains : but it is not by any means true that this 

 action always accounts for the absence of organic remains from the older strata. 



4. The ancient separation of the old schistose deposits into two great groups 

 (the upper containing organic remains, and the lower not,) appears, therefore, 

 to be borne out by the phenomena of our Island. This separation is, how- 

 ever, on two accounts, of very little importance. First, Because the two 

 groups almost entirely interchange mineralogical characters. Secondlj/, Be- 

 cause, between the two extremes, there appears to be an enormous develop- 

 ment of rocks, in which organic remains (though appearing here and there) 

 are very rare. Hence it must always be difficult, and oftentimes impossible, 

 to draw any precise line between the two groups in question. 



5. I believe, however, that there is a broad mineralogical distinction be- 

 tween the primary stratified rocks (including under that term all stratified 

 rocks inferior to the old red sandstone,) and the secondary, — and a still broader. 



